Coatings are commonly applied to particulate materials as, for example, may be required to control the rate of release of an active ingredient in the material over an extended period. One such application for such coatings is slow-release fertilizers in which the release rate of the fertilizer is controlled in order to extend the period of time over which the active ingredients, i.e., the nutrients, are delivered.
Some commercial fertilizers, of which urea is a typical example, are water soluble and dissolve rapidly when in contact with water. Consequently, when fertilizers of this type are employed for agricultural or horticultural purposes most of the nutrients are rapidly dissolved when the fertilizers are placed in contact with moisture present in the soil. It is well known that the rate of release of nutrients from the affected fertilizers can be extended, and even controlled, by enveloping the fertilizer particles in a coating suitable for the purpose. These fertilizers are referred to as "slow release fertilizers" or "controlled release fertilizers" and are used extensively on lawns, gardens and on horticultural and agricultural crops. Coating of particulate materials is also useful in applications other than fertilizers, such as for example in the pharmaceutical industry for obtaining slow release of orally administered medicaments.
In the present specification, although reference is made throughout to the coating of fertilizers, it should be understood that the same technique can be used for the coating of other active ingredients.
When particles of fertilizer are coated for the above-stated purpose, it is desirable, in order to provide a consistent, controlled release of nutrients, that the thickness of the coating be substantially uniform. Also, because the coating material dilutes the fertilizer thereby decreasing the amount of plant nutrients per unit weight of coated product, it is necessary to keep the layer or layers of coating materials applied to the particles as thin as possible. The requirements for the production of the coated fertilizer product not only increase the production cost of the concerned fertilizer product, but also add to the costs involved with transportation, storage and application of the fertilizer material.
Attempts have been made to improve the cost effectiveness of coating particulate materials. U.S. Pat. No. 3,241,520 granted Mar. 22, 1966 to D. E. Wurster, et al., for example, describes an apparatus in which a plurality of layers of the same or different coating materials are applied to particles during sequential flow through a plurality of coating and subsidence zones occupying a series of cells. According to the teaching of this patent, the particles to be coated are entrained in an air stream conducted through a diffused spray of the dissolved coating material in the coating zone of a cell. After coating, the particles are deposited in the subsidence zone of the cell to await continued movement into the succeeding cell in the series. The process described in this patent suffers the disadvantage that there can be no particle recirculation through the spraying zones in the respective cells. Although the patent describes embodiments of the invention that contemplate recirculation of particles, such teachings are limited to batch-type operations of the unit.
An alternative manner of multi-cell production of coated particles is described in U.S. Pat. No. 5,211,985 granted May 18, 1993 to A. R. Shirley, Jr. et al. and assigned to ICI Canada, Inc. This patent discloses an apparatus and process for continuously producing polymer-coated particles in a plurality of series-connected fluidized beds, in each of which the particles are conducted essentially randomly through a coating material spray. The production system disclosed in this patent is deficient in that it does not permit a controlled application of coating material to the substrate particles. Accordingly, an accurate control of the release rate of the particulate material cannot be obtained.
Consequently, while both of these patents illustrate methods and apparatus for coating particles, neither of the patents teaches or suggests methods and apparatus suitable for the cost effective manufacture of a coated product in which the coating is essentially uniform in thickness, thereby leading to a coated product with a controlled release of the enclosed ingredients. It is to the amelioration of this problem, therefore, to which the present invention is directed.